The present invention relates to a semiconductor integrated circuit device and, more particularly, to a technology suitably used for a single-chip microcomputer incorporating a programmable ROM (Read Only Memory) and having a PROM (Programmable Read Only Memory) mode for writing data to the programmable ROM.
As is known, there is a microcomputer of single-chip type having a CPU (Central Processing Unit) of stored program type, a ROM for storing a program, a RAM (Random Access Memory) for storing data, and input/output circuits all formed on a same semiconductor substrate, thereby reducing system production cost while enhancing system reliability. It is also known that replacing a ROM built in such a microcomputer with a programmable ROM such as an EPROM (Electrically Programmable ROM) and providing the PROM mode for writing the programmable ROM by means of a general-purpose ROM writer allow a user to write and/or modify programs, shortening system development period. To this end, the microcomputer is linked to the ROM writer through a predetermined socket adaptor to put activation control signals in a predetermined combination, which in turn selectively puts the microcomputer in the PROM mode.
The single-chip microcomputer incorporating the programmable ROM and having the PROM mode is described in "H8/320, H8/325, H8/324, H8/323, And H8/322 Series Hardware Manual" of Hitachi America, Ltd., December 1990, pages 1 through 17 and 197 through 207, by way of example. The single-chip microcomputer incorporating the EPROM is also disclosed in Japanese Patent Laid-open Nos. JP-A-61-51695 (U.S. Pat. No. 4,701,886) and JP-A-62-99856 (U.S. Pat. No. 4,807,114).